Penicillin tolerance in group B streptococci isolated from infected neonates.

Four (4%) of 100 isolates of group B streptococci from the blood or cerebrospinal fluid of infected neonates were shown to be tolerant to penicillin by significantly slower rates of killing at penicillin levels that were 16 times greater than the minimal inhibitory concentrations. Different killing rates between nontolerant and tolerant streptococci were observed with penicillin only in the logarithmic phase of growth. Nontolerant strains in the stationary phase and tolerant strains in both the logarithmic and stationary phases were killed at similar rates, which were slower than the rates for nontolerant streptococci in the logarithmic phase. When incubated with [14C]lysine-labeled preparations of cell wall, freeze-thaw extracts of tolerant strains released less radioactivity than did those of nontolerant strains. This activity of nontolerant streptococci was maximal during logarithmic growth and was heat-labile, a result which presumably reflected the activity of autolytic enzyme(s).